This invention pertains generally to optically fed antenna systems and in particular to an antenna of such type having a folded feed wherein aperture blockage is eliminated.
Optical or space feeds for lens array antenna systems are preferred in many applications because such feeds suffer significantly less loss than equivalent corporate feed networks and are less costly to build. Further, the relatively low weight of optically fed antenna systems makes such systems attractive for use in aircraft. However, in some applications (especially in aircraft installations) sufficient space is not always available to allow placement of the feed at the focal point of the array as is necessary for satisfactory operation. Consequently, only optically fed arrays with small focal length to diameter (F/D) ratios have been utilized in many applications. Unfortunately, with very small F/D ratio, the size of the nominal focal spot becomes very small. When the focal spot is small, good array illumination control is difficult to attain without intentionally moving the feed off the focal plane. An off focal plane feed may be just as effective as a focal plane feed in a monopulse antenna, provided both sum and difference main lobes are subtended over the frequency range of interest for the maximum scan condition of the main lens assembly. Unfortunately, however, an off focal plane feed greatly increases the complexity of the antenna system installation, especially in aircraft applications.
A known alternative to an off focal plane feed to mitigate the problems inherent with a small F/D ratio is a folded feed technique. The most effective feed using such a technique has "polarization-twist" elements to reduce the depth of the feed to one-third focal length. In such a folded feed, a horizontally-polarized multi-mode horn (or similar feed) is positioned in the center of a polarization-twisting surface. Vertically polarized energy incident on the outer face of vertically polarized antenna elements is passed to be incident on the polarization-twisting surface for conversion to horizontally polarized energy directed back toward such antenna elements for a final reflection back to the horizontally polarized multi-mode feed horn. While such an arrangement is effective in folding the optical path between antenna elements and feed, it suffers aperture blockage because the vertically polarized energy originally passed through the antenna elements and directly incident on the multi-mode feed horn is not polarization-twisted as required.